The present invention relates to the field of visual displays and image processing devices such as flat-screen computer displays, televisions, 3D displays, projection devices, time-sequential color systems and head-mount displays.
U.S. Pat. No. 4,56,758 discloses a uniformly aligned 180xc2x0 twisted surface mode liquid crystal. The pi-cell and the chirally doped variant of the pi-cell described in U.S. Pat. No. 4,566,758 are so-called surface mode liquid crystal devices. Both are characterized by having the molecules in the central region of the liquid crystal remaining aligned almost perpendicularly to the cell surfaces during operation of the device, whilst molecules close to the surfaces undergo reorientation in response to varying electric fields. However these devices suffer a disadvantage in that the optical characteristics vary non-uniformly when the device is viewed from different angles. This variation in viewing characteristics with angle is undesirable when designing, for instance, a television screen to be viewed from a wide range of angles.
GB 2 318 880 A discloses a liquid crystal device with two aligned but only one pretilted surface. However embodiments include 90xc2x0 twisted nematic rather than 180xc2x0 twisted surface mode device or pi-cell.
EP 0 750 210 A1 discloses a multi-domain liquid crystal device, in which the liquid crystal is mixed with some amount of polymer but no chiral dopant and is preferably 90xc2x0 twisted.
EP 0 768 560 A1 discloses a mult-domain liquid crystal device in which one surface has vertical alignment.
EP 0 632 311 A1 discloses a multi-domain liquid crystal device which allows an amount of chiral dopant giving d/p less than 0.75, but does not describe the operation and viewing performance of a 180xc2x0 twisted surface mode device as in the present invention.
U.S. Pat. No. 5,710,609 describes a 180xc2x0 twisted liquid crystal device which includes a liquid crystal of negative dielectric anisotropy.
U.S. Pat. No. 5,504,604 describes a multi-domain liquid crystal device which requires first and second liquid crystal layers with first and second pre-tilt angles.
U.S. Pat. No. 4,635,051 discloses the use of pi-cells as switchable optical retardation elements. The pi-cell is a surface switching device which utilizes a positive dielectric anisotropy liquid crystal confined between two surfaces which have been treated such as by rubbing in such a way as to induce unidirectional alignment of the liquid crystal with a small tilt (typically 2-5xc2x0) of the molecules away from the surface (xe2x80x98pretiltxe2x80x99).
P. D. Berezin et. al., Sov.J.Quant.Electron Vol. 3(1) pp.78-79 (1973) discloses the principle of liquid crystal surface mode switching in the form of a voltage-tunable birefringent waveplate. A layer of nematic liquid crystal is arranged with its optic axis at 45xc2x0 to the orthogonal polarizing directions of two polarizers. The device is operated between voltages such that significant molecular switching only occurs near the surfaces of the liquid crystal layer.
EP 0 616 240 A1 describes the use of a fixed uniform optical retardation film with optic axis crossed with respect to the rubbing direction of a pi-cell.
FIGS. 1A to 1D of the accompanying drawings illustrate diagrammatically the construction and operation of a typical pi-cell. The cell comprises a substrate 1 of transparent glass or plastic coated with a transparent electrode 2, for instance of indium tin oxide (ITO). The electrode 2 is coated with a thin layer 3 of a polymer, such as polyimide, which is unidirectionally rubbed or buffed so as to define the alignment direction and pretilt angle for nematic liquid crystal molecules 4. The opposite cell wall comprises a substrate 11, an electrode 12 and an alignment layer 13 of the same type as the substrate 1, the electrode 2 and the alignment layer 3, respectively. The cell walls are spaced apart by spherical glass or plastic spacers (not shown) to define a cell thickness which is typically between 1 and 25 micrometers. The cell walls are aligned with their rubbing directions parallel and the cell is sealed so as to contain a layer of liquid crystal material of positive dielectric anisotropy.
The pi-cell is operated with its molecules in a xe2x80x9cbend configurationxe2x80x9d. Above a certain voltage between the electrodes 2 and 12, typically of the order of 2 volts RMS, the liquid crystal director favors the adoption of the bend-configuration with the molecules in the central region 5 of the cell being aligned along the applied electric field. At higher fields, the bend-configuration distorts as more of the molecules align along the direction of the applied field. FIG. 1A illustrates a bend-configuration at a typical operating voltage whereas FIG. 1B illustrates the further distortion which occurs with increased operating voltage. Thus, in the bend-configuration, only the surface regions 6 distort when the applied voltage is varied whereas the central region 5 remains substantially static.
Because liquid crystal materials are optically anisotropic or xe2x80x9cbirefringentxe2x80x9d, when the pi-cell is disposed between two polarizers with the surface alignment directions of the cell oriented at 45xc2x0 to the absorption axes of the polarizers, a variation in optical transmission accompanies distortions in the liquid crystal director structure. Thus, a voltage-tunable optical shutter can be produced and can form the basis of a display device. Usually, when the pi-cell is operated as a transmissive device, a range of operating voltages is selected such that, at a lower operating voltage, the pi-cell exhibits approximately a half-wave optical retardation at a predetermined wavelength (normally approximately 550 nanometers). The device or picture element then appears bright. At an upper operating voltage, the pi-cell exhibits approximately zero optical retardation and so looks dark between the crossed polarizers. Switching between the upper and lower operating voltages produces optical switching between bright and dark states, with intermediate voltages producing intermediate grey levels.
It is known that, if a voltage is applied to a pi-cell in its bend configuration and the voltage is then reduced towards zero, below a certain level which is typically about 2 Volts, a 180xc2x0 twisted director configuration appears briefly in the cell as illustrated in FIG. 1D. This twisted structure is typically replaced shortly thereafter by the nucleation and growth of the splay configuration.
JP 9-90432 discloses twisted devices with twists of substantially 360xc2x0.
WO 97/12275 discloses another type of surface switching liquid crystal device using two parallel aligned surfaces. However, in this case, a relatively high pretilt, typically between 80xc2x0 and 90xc2x0, is produced by the alignment layers and the liquid crystal material is of the negative dielectric anisotropy type.
The 180xc2x0 twisted surface mode discussed herein is not to be confused with the 180xc2x0 twisted so-called STN device disclosed, e.g., in UK Patent GB 2 123 163B. The STN is a device designed specifically to exhibit a very sudden change in optical transmission with voltage. This allows it to be used with so-called xe2x80x9cmultiplex drive schemesxe2x80x9d and it does not use an active matrix of, e.g., thin film transistors. The surface mode devices described herein require an active matrix as discussed later in connection with FIG. 12. A disadvantage of the STN device is that its switching speed is relatively slow. In particular, switching between states typically takes more than 100 milliseconds whereas the switching times of typical surface mode devices are on the order of 10 milliseconds. Thus, STN devices are unsuitable for use as video rate displays or in other applications where switching speeds of 20 milliseconds or less are required.
The present invention relates to surface mode nematic liquid crystal devices with improved angular viewing characteristics. Conventional liquid crystal devices consist of a layer of liquid crystal confined between two glass or plastic substrates. Conventionally these substrates are uni-directionally rubbed or otherwise treated to cause the liquid crystal molecules in contact with the substrate to uni-directionally align. We disclose herein a surface mode device with surfaces treated so as to provide more than a single alignment direction of the liquid crystal (i.e. to produce multi-sub-region or multi-sub-pixel alignment) within a single pixel of the liquid crystal device which is beneficial in improving angular viewing characteristics.
Multi-sub-region alignment of a surface mode device is used to produce a device which is both faster switching than the well-known 90xc2x0 twisted nematic device, making it more suitable for the display of moving video images, and which has a more uniform viewing characteristic.
The term xe2x80x9csurface mode liquid crystal devicexe2x80x9d as used herein means a liquid crystal device in which the direction of the liquid crystal directors in a middle portion of the liquid crystal layer remote from the surface portions of the liquid crystal layer adjacent alignment layers does not change substantially when the applied field across the liquid crystal layer varies over the operating range of the device.
In one embodiment, the present invention relates to a surface mode liquid crystal device, including a liquid crystal layer disposed between first and second alignment layers for substantally parallel-aligning the liquid crystal layer, the first alignment layer including a plurality of regions defining respective picture elements, each of the regions including a plurality of sub-regions, the plurality of sub-regions each comprising at least one first sub-region for aligning the adjacent liquid crystal in a first alignment direction and at least one second sub-region for aligning the adjacent liquid crystal in a second alignment direction substantially different from that of the first alignment direction.
In one embodiment, in the first alignment layer, the plurality of sub-regions comprises at least two sub-regions, the sub-regions having alignment directions arranged in multiples of 90xc2x0 with respect to each other. In one embodiment, each plurality of sub-regions comprises four sub-regions. In one embodiment, in both the first and second alignment layers, the plurality of sub-regions comprises at least a first pair and a second pair of sub-regions, the sub-regions in each pair having alignment directions arranged in multiples of 90xc2x0 with respect to the alignment directions of adjacent sub-regions. In one embodiment, the plurality of sub-regions comprises more than four sub-regions.
In one embodiment, the first alignment layer is arranged to provide a non-zero pretilt.
In one embodiment, the second alignment layer is arranged to align the adjacent liquid crystal with a substantially zero pretilt and the liquid crystal layer is sufficiently chiral to stabilize a substantially 180xc2x0 twisted liquid crystal director configuration in the absence of an applied field across the liquid crystal layer.
In one embodiment, the liquid crystal layer comprises a nematic liquid crystal and a chiral dopant.
In one embodiment, d/p is greater than or equal to substantially 0.25, where d is the thickness of the liquid crystal layer and p is the pitch which the chiral liquid crystal would have in an unconstrained infinitely thick layer. In one embodiment, d/p is less than 0.75. In one embodiment, d/p is substantially equal to 0.25.
In one embodiment, the liquid crystal layer has a positive dielectric anisotropy.
In one embodiment, the first and second alignment layers are arranged to provide a pretilt of between 1xc2x0 and 10xc2x0. In one embodiment, the first and second alignment layers are arranged to provide a pretilt of between 1 and 5xc2x0.
In one embodiment, the first alignment layer is arranged to provide a pretilt of between 1xc2x0 and 10xc2x0. In one embodiment, the first alignment layer is arranged to provide a pretilt of between 1xc2x0 and 5xc2x0.
In one embodiment, the device includes an active matrix addressing arrangement.
In one embodiment, the liquid crystal layer is disposed between first and second polarizers whose polarizing directions are orthogonal and one of whose polarizing direction is aligned substantially at 45xc2x0 to the alignment direction or the first alignment direction of the first alignment layer.
In one embodiment, the liquid crystal layer is disposed between first and second polarizers whose polarizing directions are substantially parallel and aligned substantially at 45xc2x0 to the alignment direction or the first alignment direction of the first alignment layer.
In one embodiment, the polarizers are patterned into a plurality of sub-regions with a laterally varying direction of the polarizing axes such that the polarizing axes of each polarizer sub-region lie at 45xc2x0 to the alignment direction of the liquid crystal sub-region immediately adjacent to the polarizer.
In one embodiment, the liquid crystal layer is disposed between a polarizer and a reflector. In one embodiment, the polarizer is patterned into a plurality of sub-regions with a laterally varying direction of the polarizing axes such that the polarizing axes of each polarizer sub-region lie at 45xc2x0 to the alignment direction of the liquid crystal sub-region immediately adjacent to the polarizer.
In one embodiment, the device includes a first fixed retarder for reducing the retardation of the liquid crystal layer. In one embodiment, the first fixed retarder is of positive birefringence and has an optic axis substantially perpendicular to the alignment direction or the first alignment direction of the first alignment layer. In one embodiment, the first fixed retarder is of negative birefringence and has an optic axis substantially parallel to the alignment direction or the first alignment direction of the first alignment layer. In one embodiment, the device includes a second fixed retarder of negative birefringence having an optic axis substantially perpendicular to the liquid crystal layer.
In one embodiment, the retarder used for reducing the retardation of the liquid crystal layer comprises a plurality of regions with optic axis in each region aligned perpendicular to the alignment direction of the adjacent liquid crystal region.
In one embodiment, the first alignment direction and the second alignment direction lie at a non-integral multiple of 90xc2x0 with respect to each other.